Removal of solvent from cellulosic plastics



Patented Oct. 29, 1935 UNITED STATES PATENT OFFICE REMOVAL OF SOLVENT FROM CELLULOSI PLASTIC poration of Delaware No Drawing. Application January 24, 1928, Serial No. 249,201

13 Claims.

This invention is in the art of cellulosic plastics and has particularly to do with a method of removing volatile solvent from such plastics, such removal of solvent being necessary, in the manufacture of plastics, in order to place them in condition to be worked in the usual fabricating processes, such as shaping articles from the stock, and so forth. While the invention is, for deflniteness, described with more particular reference to cellulose nitrate plastics, it is to be'understood that the invention is not restricted to plastics having cellulose nitrate as a base, as it also has application to the recovery of solvent from plastics having other cellulosic compounds as a base, such as other cellulose esters, e. g. cellulose acetate, and cellulose ethers.

We have found that, although a water treatment of cellulose nitrate having a nitrogen content of over approximately 12.5 per cent, and not plasticized with a non-volatile plasticizer, such as camphor or a camphor substitute, develops permanent porosity and milkiness in the nitrocellulose, such is not the fact when nitrocellulose having a nitrogen content below 12.5 per cent, say between 10.4 per cent and 12.2 per cent, and plasticized with a substantial quantity, say 8 per cent to 40 per cent, of a relatively non-volatile plasticizer, e. g. camphor, aromatic phosphates, etc., is water treated. Our invention is basedon this discovery. A major object of our invention is to provide a method for the removal of volatile solvent from cellulosic plastic, whereby the solvent may be removed with much greater rapidity, and at the same time more uniformly, than has heretofore been possible.

In general, the present solvent recovery or seasoning method comprises the treatment of the plastic with water to remove a considerable portion of the volatile solvent by diffusion of such solvent into the water, it being understood that the volatile solvents ordinarily used are soluble in water; and the treatment further comprises the removal of additional solvent by air drying after the water treatment. Preferably, the plastic is subjected to a preliminary air drying previous to the water treatment.

The removal of solvent by the water and air treatment presents a substantial advantage over the usual method of solvent removal by air treatment only. When subjected to this usual air drying treatment, the solvent is first removed from the surface of the plastic, say a sheet, which "case hardens the surface, and this greatly retards complete evaporation of the solvent from the interior portion of the plastic; and, for this reason, air-seasoned plastic has a substantial non-uniformity of (residual) solvent content, and furthermore the time of seasoning is very long, sometimes being as much as 12 months.

By the present method, these, and other, difi'iculties are eliminated, since the water treatment keeps the surface of the plastic soft and allows the solvent to diffuse into the water, this action continuing to the innermost portions of the plastic. Furthermore, upon the air drying, subsequent to the water treatment, the water absorbed by the plastic, together with the (diluted) solvent still in the plastic, quickly evaporates from the interior and the surface of the plastic and leaves the plastic in a very dry condition, the reduction 15 of the solvent content to the desired residual amount being thus very quickly obtained.

As indicated above, an air treatment prior to the Water treatment is preferable. When plastic is first subjected to a seasoning treatment, it contains a large proportion of solvent, say 8 to 16 per cent, and a substantial portion of this content can be readily and quickly removed by air treatment, thus avoiding the use of excessive amounts of water, which would be necessary if a water treatment were used to remove all of the solvent intended to be removed prior to the final air drying. For practical purposes, an initial air treatment may be advantageously used to remove approximately half of the solvent content, thus reducing the solvent content to 4 per cent to 8 per cent. As will be understood by those skilled in the art, the extent to which an initial air treatment may be advantageously used will depend upon the initial solvent content of the 85 plastic, the thickness of the plastic, e. g. sheets, being treated, and like factors, but it can, of course, be readily determined for each individual case as such case arises, care being taken that it is not carried far enough to result in case 40 hardening to an objectionable extent.

By way of example merely, and not by way of restriction, plastic for treatment may be sheet stock as sheeted from a block resulting from the regular cakepressing operation, and containing,--

Parts by weight Pyroxylin (N content 11%) 66 Camphor 22 Ethyl alcohol (denatured) 12 Sheeting Prelim- 1 T t 1 thickinary Water ness air Inch Days Dal s Days Days In this treatment, in each case, the solvent is reduced by the preliminary air treatment to about 6%, is further reduced by a substantial amount by the water treatment, and by the air treatment is yet further reduced, say to about two per cent, the water treatment not only removing solvent but also diluting the solvent remaining in the plastic and thus rendering the removal of the solvent more rapid in the subsequent air treatment for the removal of the water and the reduction of the solvent to the desired residual amount. For the same final solvent content of the plastic, approximately twice the times above given is required when the usual method (using air treatment only) is practised.

.In general, the amount of solvent removed at each step will vary with the times of treatment, initial solvent content, and so on. Approximately it may be reduced to 4 to 8 per cent by the first air treatment, is further reduced by the water treatment, and to 1 to 2 per cent by the second air treatment.

In practising the present method, the plastic material, for example sheets, is suspended by means of strings or hooks, or placed in any other suitable manner, in rooms or boxes maintained at the desired temperature, and the preliminary air treatment given. Next the plastic is transferred to a suitable tank filled with water and provided with a closed steam coil for heating. The temperature may be regulated by an operator or by an automatic control. The time of immersion in the water is determined by the rate of penetration of the water. It is desirable that the water thoroughly penetrate the plastic, e, g. penetrate to the center of the plastic from each side, and this is evidenced, in transparent material, by clouding. After the requisite water treatment, the plastic is returned to the heated rooms or boxes, where the evaporation is completed. Completion of the seasoning may be readily determined, for practical purposes, by the disappearance of the clouding caused by the water treatment, but it may be determined in other ways, for example by testing the solvent content of samples or by weighing the plastic at intervals and noting when it ceases to lose weight. (The weight of the plastic during the water treatment remains practically unchanged since the solvent removed is replaced by water.)

The diffusion of solvent into the water results in a gradually increasing solvent content of the water. If this be allowed to become too high, the rate of diffusion will be measurably slowed down. In practice, when using tanks of a size substantially larger than the charge, and filled with water, it has been found sufilcient to change the water after each charge of plastic has been treated.

While particular mention of treating sheets has been made above, it is obvious that other forms, such as tubes or rods, for example, may also be treated. In determining seasoning time for tubing, the thickness of wall may be considered as equivalent to the thickness of a sheet. Rod diameter may be considered as equivalent to half sheeting thickness, 1. e. a rod one-half inch in diameter would require the same seasoning time as sheeting one-fourth inch thick.

As previously mentioned, instead of a treatment comprising a preliminary air treatment, there may be used a treatment embodying only a water treatment and a following air treatment. Satisfactory and rapid removal of solvent can be obtained in this manner, and such a procedure, of course, embodies our invention of using a water treatment followed by an air treatment, but the omission of the first air treatment results in more alcohol passing into the water, which makes desirable frequent changing of the water so as to prevent the building up in the water of an alcohol content so high as to retard the solvent removal. Using a temperature of 45 C. for both the water and the air treatment, the following schedule has been found to give proper removal of solvent.

The times given in this table, and in the table preceding, result in a residual solvent in the finished plastic which enables it to be worked in the usual fabricating processes, considerably less than'the original amount of solvent remaining after the water treatment, and yet less (about 1 to 2 per cent) remaining after the air treatment. It must not be concluded, however, that these times are absolute; they may be varied in several ways without greatly affecting the result. For example in the table preceding, where a succession of air, water and air treatments is given, the first air treatment may be reduced and this added to the final air treatment. The water treatment in both tables may be increased several days without affecting the result materially. As indicated above, when the preliminary air treatment is omitted, frequent changes of water are desirable, the number of these advisable, in order to prevent the alcohol content becoming so high as to objectionably retard the rate of solvent removal, depending upon the circumstances of each particular case and being readily determinable, of course, by those skilled in the art, as necessity arises.

The times may also be varied to give less complete or more complete removal of the solvent, but such varied times will always be approximately one-half those required for the same thickness of sheet in air seasoning and for an equal degree of solvent removal. In varying the times, two points should be considered; first, that the time of water treatment should be long enough for the water to fully penetrate the plastic, and, second, that the final air treatment must be long enough to clear the stock from any opacity resulting from the water treatment. It is permissible to vary the temperature from C. or even lower is therefore preferable.

45 C. indicated as the usual and preferable tem-= perature. The practical range is 25C. to 70 C. Usually delicately colored plastic will be seasoned at a temperature (both' water and air) below 45 0., say 35 C. Plastic rods tend to blister or lose their circular cross section; a temperature of 35 The seasoning time would, of course, be varied inversely with the temperature, 1. e., for a given thickness the time would be longest with the minimum temperature and shortest with the maximum temperature.

If the plastics contain an essential water-soluble ingredient, such as stabilizer, the constancy of this ingredient may be insured by dissolving enough in the water to give equilibrium between the substance in the plastic and in the water.

Methods of solvent recovery for the recovery of solvent removed from the plastic by the present method are, of course, usable. Thus, the solvent in the solvent-laden air from the preliminary and final air treatments may be condensed lengths of plastic films such as those made by the. casting process. In the latter case the film would be passed successively through the various treatments and finally wound in the form of a roll. Since the thickness of film is small compared to sheeting previously referred to, the times would be correspondingly shorter.

' We claim:

l. The method of removing water-soluble volatile solvent from a cellulosic mass containing nitrocellulose of a nitrogen content substantially 1 between 10.4 and 12.2%, such solvent and a plasticizer which method comprises, subjecting the mass to a preliminary air treatment, then to a water treatment, and then to a final air treatment.

2'. The method of removing water-soluble volatile' solvent from a pyroxylin mass containing,

nitrocellulose of a nitrogen content substantially between 10.4 and 12.2 per cent, water-soluble volatile solvent, and plasticizer substantially between 8 and 40 per cent, which method comprises, subjecting the mass to a preliminary air treatment, then to a water treatment, and then to an air treatment, such treatments being performed at substantially between 25 and 70 C. 3. The method of removing water-soluble volatile solvent from a pyroxylin mass containing, nitrocellulose of a nitrogen content substantially between 10.4 and 12.2 per cent, volatile solvent substantially between 8 and 16 per cent, and plasticizer substantially between 8 and 40 .per cent, which method comprises, subjecting the mass to a preliminary air treatment at approximately 45" C. until the solvent content is reduced to about 4 to 8 per cent, then subjecting,

the mass to a water treatment at approximately 45 C. until the water has fully penetrated the plastic and the solvent content is further reduced, and then subjecting the mass to a final airtreatment at approximately 45 C. until the water is removed from the mass and the solvent content is reduced to the desired residual amount, e. g. 1 to 2 per cent.

4. In the seasoning of a cellulosic plastic or the cellulose-derivatlve-and-plasticizer type, and containing nitrocellulose of a nitrogen content substantially between 10.4 and 12.2% and a plasticizer combined therewith, and also containing a water-soluble volatile solvent to be removed therefrom by seasoning, the same being in the 10 general character of an integrated mass as distinguished from a free-flowing solution, with a solvent content consistent with such character; subjecting the plastic to a water treatment, prolonged to remove a substantial proportion of said solvent by diffusion thereof into the water, and afterwards subjecting the plastic to an air treat,- ment.

5. In the seasoning of a cellulosic plastic of the cellulose-derivative-and-plasticizer type, and containing nitrocellulose of a nitrogen content substantially between 10.4 and 12.2% and a plasticizer combined therewith, and also containing a water-soluble volatile solvent to be removed therefrom by seasoning, the same being in the general character of an integrated mass as distinguished from a free-flowing solution, with a solvent content consistent with such character: subjecting the plastic to a water treatment, at substantially between 25 C. to 70 C., prolonged to remove a substantial proportion of said sol-- vent by diffusion thereof into the water, and afterwards subjecting the plastic to an air treatment.

6. In the seasoning of a cellulosic plastic of the cellulose-derivative-and-plasticizer type, and containing nitrocellulose of a. nitrogen content substantially between 10.4 and 12.2% and a plasticizer combined therewith, and also containing a water-soluble volatile solvent to be removed therefrom by seasoning, the same being in the general character of an integrated mass as distinguished from a free-flowing solution, with a solvent content consistent with such character: subjecting the plastic to a water treatment, prolonged to remove a substantial proportion of said solvent by diffusion thereof into the water, and afterwards subjecting the plastic to an air treatment at substantially between 25 C. to 70 C.

'7. In the seasoning of a cellulosic plastic of the cellulose-derivative-and-plasticizer type, and containing nitrocellulose of a nitrogen content substantially between 10.4 and 12.2% and a plasticizer combined therewith in an amount substantially between 8 to 40%, and also containing a water-soluble volatile solvent to be removed therefrom by seasoning, the same being in the general character of an integrated mass as distinguished from a free-flowing solution, with a solvent content consistent with such character:

subjecting the plastic to a water treatment, prolonged to remove a substantial proportion of said solvent by diffusion thereof into the water, and afterwards subjecting the plastic to an air treatment. 8. That method of curing pyroxylin plastic material which comprises subjecting the pyroxylin plastic material for a time to water at a temperature substantially between 25 C. and C., and then drying the wet plastic material 70 for a time in an atmosphere of air at a temperature substantially between 25 C. and 70 C., the time of treatment increasing according to the thickness of the material, but varying inversely to the temperature, the temperature for delicate colors being within the lower portions or said ranges, and the total time of treatment being materially less than the present regular seasoning time.

9. That method of curing pyroxylin plastic material which comprises subjecting the pyroxylin plastic material for a time to water at a temperature substantially between in the neighborhood of C. and C., and then drying the wet plastic material for a time in an atmosphere of air at a temperature substantially between in the neighborhood of 35 C. and 70 (3., the time of treatment increasing according to the thickness of the material, but varying inversely to the temperature, the temperature for delicate colors being within the lower portions of said ranges, and the total time of treatment being materially less than the present regular seasoning time.

10. That method of reducing the length of time required in curing pyroxylin plastic material, which consists in first subjecting the pyroxylin plastic material to water at a temperature of about to F. according to the stable qualities of colors in the material and the thickness of the material for a period approximating one quarter of the present seasoning time, then drying the wet plastic material for approximately a second quarter of the regular seasoning time in an atmosphere of air at from 90 to 130 F. proportionately to the stable qualities of the colors in the plastic material and the thiekneq of the latter.

11. The process of seasoning sheet pyroxylin plastic which consists in forming the pyroxylin plastic material into a sheet while still contain- 5 ing the original solvent, removing a portion .0! the solvent by exposure to the air until the surface of the sheet is set in its normal unclouded condition and then removing more oi the solvent by immersing the sheet in water. 10

12. The process of seasoning sheet pyroxy plastic which consists in iorming the pyronlin plastic material'into a sheet while still containing the original solvent, removing a portion 01' the solvent by exposure to the air until the surll face 01' the sheet is set in its normal unclouded condition and then removing more of the solvent by immersing the sheet in hot water.

13. The process of seasoning sheet pyroxylin plastic which consists in forming the pyroxylin plastic material into a sheet while still containing the original solvent, removing a portion oi the solvent by exposure to the air until the surface of the sheet is set in its normal unclouded condition and then removing more of the solvent by immersing the sheet in water at a temperature of about F.

. JOHN H. CLEWELL.

FLOYD V. WEMPLI. l0 

